The invention relates to a device for monitoring a melt for the production of crystals and a process therefor.
When a given material, for example, Si polymaterial is thoroughly melted in a quartz crucible, the transition from a solid to a liquid state does not, as a rule, occur abruptly, but rather, gradually. In the actual practice of creating crystals it is very important that the given melt state be precisely known or identifiable, since this state dictates the procedural steps that are to follow.
There are experienced melt specialists who are able to precisely infer the state of the melt from the surface appearance of a crucible. Also known is the automatic identification of a melt state using pyrometers as sensors, though this results in an unreliable kind of regulation involving long time constants.
Various processes have already been suggested for an improved automatic regulation of the drawing process of a monocrystal. For example, a process for drawing a monocrystal from the melt is known in which the individual crystals are drawn up while the data influencing the drawing process, which are based on numerous conditions, are recorded and compared with other data (EP 0 536 405 A1). In the process, a laser beam, for example, strikes the surface of molten material located in a crucible. Identification of a reflected laser beam allows the position of the molten surface to be determined, and the crucible is elevated according to the difference between the measured position and a predetermined position. However, this known process does not permit reliable process monitoring during the melting phase.
Also known is an optical system or process for regulating the growth of a silicon crystal in which the diameter of a silicon crystal drawn from a melt is measured with the assistance of a television camera; here the surface of the melt exhibits a meniscus which is visible as a bright area in close proximity to the silicon crystal (EP 0 745 830 A2). In this system, a camera is used to produce an image pattern of a portion of the bright area near the silicon crystal. The characteristics of the image pattern are then detected. A characteristic of the visual pattern would be, for example, the light intensity gradient. After this, one edge of the bright area is defined as a function of the detected characteristics. Then an outline is defined which closes the defined edge of the bright area; finally the diameter of the defined outline is determined, and the diameter of the silicon crystal is determined as a function of that defined outline diameter. A disadvantage of this known system is that its accuracy is insufficient in several applications and external influences, in particular, are not adequately taken into account.
To eliminate these disadvantages it has already been suggested to add to the camera imaging the first area of the crystal a second camera which images a second portion of the crystal; the diameter of the crystal is determined in an evaluation circuit using the images of both cameras (unpublished patent application 197 38 438.2). In this manner it is possible to precisely record the actual crystal diameter in all phases of the cultivation process. The melting process itself cannot be monitored with the proposed device.
An object of the invention is to overcome the problem of monitoring the melting process for the basic materials from which monocrystals are drawn.